SJ CT Coronary Technique

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How to Monitor Coronary CT On Call at St Joseph

Medications

One key for getting a good study is low, steady heart rate ~ 60. Most patients should get 50 mg of Metropolol (Lopressor) orally 1/2 hour before study. Good heart rates can go bad with the anxiety of the study and contrast injection so I end up giving the beta blocker to almost everyone who can be expected to tolerate it.

Injection

I like to be in the room as another pair of eyes

Nothing is more critical than having the patient injection arm up in the air to facilitate contrast moving into the SVC. You cannot "push" contrast with saline very well through arm vein if the arm is down. It will simply float over the top of the contrast.

At St Joseph we use 65 cc of a contrast injection at 5 cc a second followed by 40 cc of dilute contrast (40% contrast / 60% saline) so as to not have too much density in the right heart. That gives you about 13 seconds of a real contrast bolus. Effectively, the total contrast that the patient gets is 80 cc. The imaging time is only going to be about 9 seconds so it will work if the timing is reasonable. The techs wait until the first glimpse of contrast in the aorta and then hit GO button. Scanning on the GE will start about 6 seconds later.

The dilute contrast is given for two reasons:

  1. To push the dense contrast remaining in the arm into the heart
  2. To make the right heart chamber have low contrast density to minimize artifacts and possibilities of obscuring the right coronary artery. This is a nice goal and is achievable but is not all that critical. For the triple rule-outs, it is impractical to have saline in the right heart and expect to have high contrast in the pulmonary artery so do not attempt to do this in that patient population.

Image timing (smart prep)

When we do CTA of the head, we view the pulmonary artery to try and judge when to push the button to start imaging. There is a 6 second delay between the time that the button is pushed and the imaging starts. If we wait for the contrast to reach the aorta, we are a little late. When the scanner reaches the head, there will be too much venous contamination. With the coronaries, this is not a problem. So the timing for coronaries is easier.

The rule is this - when you see contrast reach the aorta, push the GO button. It will take 6 seconds for imaging to start and that will work well for the coronaries.

The scanner smart prep protocol should be set up so that imaging starts at 8 seconds. Then take a picture every 2 seconds.

Position the smart prep scan at the level of the bottom of the tracheal bifurcation. This will allow you to see the aorta and the pulmonary artery. Seeing the pulmonary artery will give you an idea of how good the bolus is going to be and give you a heads up as to when the contrast will be hitting the aorta. Usually that will be 4 images (eight seconds) after contrast hits the pulmonary artery). When you see ANY contrast in the aorta, push the GO button.

Should the study be repeated? Things to check

If things went wrong AND you have reason to believe that you know why AND this thing probably will not happen again then you can repeat the study. 80 cc is not a lot of contrast. 160 cc is OK for most people. Radiation is more of a concern.

In the ER setting, if the study is of reasonable quality and the arteries look good you have some useful information. If the study is of poor quality, or the arteries are stenotic and diseased, or the case is confusing, the patient cannot be "cleared" and work-up will continue with other tests and observations. The details are not that important, and the final dictation can be left until the morning.

Triple (or double) rule-outs - with Gating

The contrast dose will be different but the TIMING is the same. Scanning with gating is a lot slower than getting a PE study without gating. If it takes 9 seconds to do a coronary only study, it is going to take about 15 seconds to do a triple rule-out with imaging from about a cm above the aorta down to below the heart.

I would use 100 cc of contrast at 4 cc per sec or 120 cc of contrast at 5 cc per sec, which is not a huge amount. At 4 cc per second, that is going to give you a 25 second bolus window, which is plenty. 25 seconds of bolus to do a 15 second study means that you do not have to be very precise.

There is no point in even trying to get the right ventricle to have saline in it when you need to have contrast in the pulmonary artery

Hit the GO button when the contrast is first seen in the aorta. This is the same as the routine coronary timing.

5 cc per sec versus 4 cc per second

If the IV is good and you choose to use 5 cc per second that is fine. The only thing you have to do is proportionally increase the volume of contrast. All the timing things are the same.

Injections via PICC lines

Start the monitoring with the smart prep at 6 seconds


Robert Livingston